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BGP MED Value

kfarrington
Level 3
Level 3

The BGP MED value can be received from one ebgp (AS1) peer to another ebgp (AS2) peer (say an internal IGP metric redist into AS1s bgp and accross to AS2). If the receiving AS2 peer is a transit network to AS3, crossing the ebgp peer to AS3, would the MED value remain intact, if not, what does it get set to? (is it zero? and why?).

Has anyone got some detailed information on this please?

Many thx :)

6 Replies 6

rwiesmann
Level 4
Level 4

Hi

the med is an optional trasitive attribute and is only exchanged between ASs. A

Med Value which comes into an AS does not leave that AS again. The med value will be reseted to zero if you pass it on to an other AS (unless you set again the outgoing med).

You can find all about bgp also in the book "internet routing architectures" from

bassam halabi. I guess you would find it on the web too.

Hope that heps

Roger

It does help :)

many thx

Hello Roger,

Is there a command or method to prevent AS from reseting MED (In fact, i would like to keep my MED end to end while passing through an AS).

Best regards

Hi

As i stated in the posting above you have to set the med value again on the outbound interface.

That means you set the med towards an other as....the med will be o.k in this as....as soon as you send it to a third as it will be reseted...so you have to set the med again outbound towards this third as.

Hope that helps

Roger

This is very true and I tested it on Friday

pls look at this

RTR4 <- RTR3 <- MSFC1 <- RTR1 <- RTR2

400 eBGP 100 100 IBGP 100 100 eBGP 200 EIGRP 11

!

router eigrp 11

network 10.0.0.0

no auto-summary

no eigrp log-neighbor-changes

!

rtr2>sh ip route

10.0.0.0/8 is variably subnetted, 14 subnets, 3 masks

D 10.10.1.0/24 [90/156160] via 10.21.21.1, 17:41:30, FastEthernet0/1

D 10.10.2.0/24 [90/156160] via 10.21.21.1, 17:41:30, FastEthernet0/1

D 10.10.3.0/24 [90/156160] via 10.21.21.1, 17:41:30, FastEthernet0/1

D 10.10.4.0/24 [90/156160] via 10.21.21.1, 17:41:30, FastEthernet0/1

D 10.10.5.0/24 [90/156160] via 10.21.21.1, 17:41:30, FastEthernet0/1

C 10.10.6.0/24 is directly connected, Loopback1

C 10.10.7.0/24 is directly connected, Loopback2

C 10.10.8.0/24 is directly connected, Loopback3

C 10.10.9.0/24 is directly connected, Loopback4

D 10.2.2.0/30 [90/284160] via 10.21.21.1, 17:41:30, FastEthernet0/1

C 10.21.21.0/30 is directly connected, FastEthernet0/1

C 10.3.3.0/30 is directly connected, FastEthernet0/0

C 10.10.10.0/24 is directly connected, Loopback5

C 10.100.100.1/32 is directly connected, Loopback100

rtr2>

rtr2>sh ip bgp

% BGP not active

rtr2>

router eigrp 11

network 10.0.0.0

no auto-summary

no eigrp log-neighbor-changes

!

router bgp 200

bgp log-neighbor-changes

redistribute eigrp 11

neighbor 10.2.2.1 remote-as 100

no auto-summary

!

rtr1>sh ip route

10.0.0.0/8 is variably subnetted, 14 subnets, 3 masks

C 10.10.1.0/24 is directly connected, Loopback1

C 10.10.2.0/24 is directly connected, Loopback2

C 10.10.3.0/24 is directly connected, Loopback3

C 10.10.4.0/24 is directly connected, Loopback4

C 10.10.5.0/24 is directly connected, Loopback5

D 10.10.6.0/24 [90/156160] via 10.21.21.2, 17:42:12, FastEthernet0/1

D 10.10.7.0/24 [90/156160] via 10.21.21.2, 17:42:12, FastEthernet0/1

D 10.10.8.0/24 [90/156160] via 10.21.21.2, 17:42:12, FastEthernet0/1

D 10.10.9.0/24 [90/156160] via 10.21.21.2, 17:42:12, FastEthernet0/1

D 10.3.3.0/30 [90/30720] via 10.21.21.2, 17:42:12, FastEthernet0/1

D 10.10.10.0/24 [90/156160] via 10.21.21.2, 17:42:12, FastEthernet0/1

C 10.21.21.0/30 is directly connected, FastEthernet0/1

C 10.2.2.0/30 is directly connected, FastEthernet0/0

C 10.100.100.1/32 is directly connected, Loopback100

rtr1>

rtr1>sh ip bgp

BGP table version is 45, local router ID is 10.100.100.1

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 10.2.2.0/30 0.0.0.0 0 32768 ?

*> 10.3.3.0/30 10.21.21.2 30720 32768 ?

*> 10.10.1.0/24 0.0.0.0 0 32768 ?

*> 10.10.2.0/24 0.0.0.0 0 32768 ?

*> 10.10.3.0/24 0.0.0.0 0 32768 ?

*> 10.10.4.0/24 0.0.0.0 0 32768 ?

*> 10.10.5.0/24 0.0.0.0 0 32768 ?

*> 10.10.6.0/24 10.21.21.2 156160 32768 ?

*> 10.10.7.0/24 10.21.21.2 156160 32768 ?

*> 10.10.8.0/24 10.21.21.2 156160 32768 ?

*> 10.10.9.0/24 10.21.21.2 156160 32768 ?

*> 10.10.10.0/24 10.21.21.2 156160 32768 ?

*> 10.21.21.0/30 0.0.0.0 0 32768 ?

*> 10.100.100.1/32 0.0.0.0 0 32768 ?

rtr1>

!

router bgp 100

no synchronization

neighbor 10.2.2.2 remote-as 200

neighbor 11.2.2.2 remote-as 100

neighbor 11.2.2.2 next-hop-self

no auto-summary

!

MSFC1#sh ip route

C 127.0.0.0/8 is directly connected, EOBC0

10.0.0.0/8 is variably subnetted, 14 subnets, 3 masks

B 10.10.1.0/24 [20/0] via 10.2.2.2, 00:21:22

B 10.10.2.0/24 [20/0] via 10.2.2.2, 00:21:22

B 10.10.3.0/24 [20/0] via 10.2.2.2, 00:21:22

B 10.10.4.0/24 [20/0] via 10.2.2.2, 00:21:22

B 10.10.5.0/24 [20/0] via 10.2.2.2, 00:21:22

B 10.10.6.0/24 [20/156160] via 10.2.2.2, 00:21:22

B 10.10.7.0/24 [20/156160] via 10.2.2.2, 00:21:22

B 10.10.8.0/24 [20/156160] via 10.2.2.2, 00:21:22

B 10.10.9.0/24 [20/156160] via 10.2.2.2, 00:21:22

B 10.21.21.0/30 [20/0] via 10.2.2.2, 00:21:22

B 10.10.10.0/24 [20/156160] via 10.2.2.2, 00:21:22

C 10.3.3.0/30 is directly connected, Vlan3

C 10.2.2.0/30 is directly connected, Vlan2

B 10.100.100.1/32 [20/0] via 10.2.2.2, 00:21:23

11.0.0.0/30 is subnetted, 2 subnets

C 11.3.3.0 is directly connected, Vlan5

C 11.2.2.0 is directly connected, Vlan4

MSFC1#

MSFC1#sh ip bgp

BGP table version is 15, local router ID is 11.3.3.1

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 10.2.2.0/30 10.2.2.2 0 0 200 ?

*> 10.3.3.0/30 10.2.2.2 30720 0 200 ?

*> 10.10.1.0/24 10.2.2.2 0 0 200 ?

*> 10.10.2.0/24 10.2.2.2 0 0 200 ?

*> 10.10.3.0/24 10.2.2.2 0 0 200 ?

*> 10.10.4.0/24 10.2.2.2 0 0 200 ?

*> 10.10.5.0/24 10.2.2.2 0 0 200 ?

*> 10.10.6.0/24 10.2.2.2 156160 0 200 ?

*> 10.10.7.0/24 10.2.2.2 156160 0 200 ?

*> 10.10.8.0/24 10.2.2.2 156160 0 200 ?

*> 10.10.9.0/24 10.2.2.2 156160 0 200 ?

*> 10.10.10.0/24 10.2.2.2 156160 0 200 ?

*> 10.21.21.0/30 10.2.2.2 0 0 200 ?

*> 10.100.100.1/32 10.2.2.2 0 0 200 ?

MSFC1#

!

router bgp 100

no synchronization

bgp log-neighbor-changes

neighbor 11.2.2.1 remote-as 100

neighbor 11.22.22.2 remote-as 400

no auto-summary

!

rtr3>sh ip route

10.0.0.0/8 is variably subnetted, 14 subnets, 3 masks

B 10.10.1.0/24 [200/0] via 11.2.2.1, 00:05:40

B 10.10.2.0/24 [200/0] via 11.2.2.1, 00:05:40

B 10.10.3.0/24 [200/0] via 11.2.2.1, 00:05:40

B 10.10.4.0/24 [200/0] via 11.2.2.1, 00:05:40

B 10.10.5.0/24 [200/0] via 11.2.2.1, 00:05:40

B 10.10.6.0/24 [200/156160] via 11.2.2.1, 00:05:40

B 10.10.7.0/24 [200/156160] via 11.2.2.1, 00:05:40

B 10.10.8.0/24 [200/156160] via 11.2.2.1, 00:05:40

B 10.10.9.0/24 [200/156160] via 11.2.2.1, 00:05:40

B 10.21.21.0/30 [200/0] via 11.2.2.1, 00:05:40

B 10.10.10.0/24 [200/156160] via 11.2.2.1, 00:05:40

B 10.2.2.0/30 [200/0] via 11.2.2.1, 00:05:40

B 10.3.3.0/30 [200/30720] via 11.2.2.1, 00:05:41

B 10.100.100.1/32 [200/0] via 11.2.2.1, 00:05:41

11.0.0.0/8 is variably subnetted, 7 subnets, 2 masks

C 11.11.1.0/24 is directly connected, Loopback1

C 11.11.2.0/24 is directly connected, Loopback2

C 11.11.3.0/24 is directly connected, Loopback3

C 11.11.4.0/24 is directly connected, Loopback4

C 11.11.5.0/24 is directly connected, Loopback5

C 11.22.22.0/24 is directly connected, Ethernet0/1

C 11.2.2.0/30 is directly connected, Ethernet0/0

12.0.0.0/24 is subnetted, 1 subnets

C 12.12.1.0 is directly connected, Loopback6

rtr3>

rtr3> sh ip bgp

BGP table version is 15, local router ID is 12.12.1.1

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*>i10.2.2.0/30 11.2.2.1 0 100 0 200 ?

*>i10.3.3.0/30 11.2.2.1 30720 100 0 200 ?

*>i10.10.1.0/24 11.2.2.1 0 100 0 200 ?

*>i10.10.2.0/24 11.2.2.1 0 100 0 200 ?

*>i10.10.3.0/24 11.2.2.1 0 100 0 200 ?

*>i10.10.4.0/24 11.2.2.1 0 100 0 200 ?

*>i10.10.5.0/24 11.2.2.1 0 100 0 200 ?

*>i10.10.6.0/24 11.2.2.1 156160 100 0 200 ?

*>i10.10.7.0/24 11.2.2.1 156160 100 0 200 ?

*>i10.10.8.0/24 11.2.2.1 156160 100 0 200 ?

*>i10.10.9.0/24 11.2.2.1 156160 100 0 200 ?

*>i10.10.10.0/24 11.2.2.1 156160 100 0 200 ?

*>i10.21.21.0/30 11.2.2.1 0 100 0 200 ?

*>i10.100.100.1/32 11.2.2.1 0 100 0 200 ?

rtr3>

!

router bgp 400

bgp log-neighbor-changes

network 11.0.0.0

neighbor 11.22.22.1 remote-as 100

no auto-summary

!

AND RTR 4

rtr4#sh ip route

10.0.0.0/8 is variably subnetted, 15 subnets, 3 masks

B 10.10.1.0/24 [20/0] via 11.22.22.1, 00:06:42

B 10.10.2.0/24 [20/0] via 11.22.22.1, 00:06:42

B 10.10.3.0/24 [20/0] via 11.22.22.1, 00:06:42

B 10.10.4.0/24 [20/0] via 11.22.22.1, 00:06:42

B 10.10.5.0/24 [20/0] via 11.22.22.1, 00:06:42

B 10.10.6.0/24 [20/0] via 11.22.22.1, 00:06:42

B 10.10.7.0/24 [20/0] via 11.22.22.1, 00:06:42

B 10.10.8.0/24 [20/0] via 11.22.22.1, 00:06:42

B 10.10.9.0/24 [20/0] via 11.22.22.1, 00:06:42

B 10.21.21.0/30 [20/0] via 11.22.22.1, 00:06:42

B 10.10.10.0/24 [20/0] via 11.22.22.1, 00:06:42

B 10.2.2.0/30 [20/0] via 11.22.22.1, 00:06:42

B 10.3.3.0/30 [20/0] via 11.22.22.1, 00:06:44

C 10.201.201.0/24 is directly connected, Ethernet1/0

B 10.100.100.1/32 [20/0] via 11.22.22.1, 00:06:44

11.0.0.0/8 is variably subnetted, 7 subnets, 2 masks

C 11.11.6.0/24 is directly connected, Loopback1

C 11.11.7.0/24 is directly connected, Loopback2

C 11.11.8.0/24 is directly connected, Loopback3

C 11.11.9.0/24 is directly connected, Loopback4

C 11.11.10.0/24 is directly connected, Loopback5

C 11.22.22.0/24 is directly connected, FastEthernet0/1

C 11.3.3.0/30 is directly connected, FastEthernet0/0

rtr4#

rtr4#sh ip bgp

BGP table version is 143, local router ID is 11.11.10.1

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 10.2.2.0/30 11.22.22.1 0 100 200 ?

*> 10.3.3.0/30 11.22.22.1 0 100 200 ?

*> 10.10.1.0/24 11.22.22.1 0 100 200 ?

*> 10.10.2.0/24 11.22.22.1 0 100 200 ?

*> 10.10.3.0/24 11.22.22.1 0 100 200 ?

*> 10.10.4.0/24 11.22.22.1 0 100 200 ?

*> 10.10.5.0/24 11.22.22.1 0 100 200 ?

*> 10.10.6.0/24 11.22.22.1 0 100 200 ?

*> 10.10.7.0/24 11.22.22.1 0 100 200 ?

*> 10.10.8.0/24 11.22.22.1 0 100 200 ?

*> 10.10.9.0/24 11.22.22.1 0 100 200 ?

*> 10.10.10.0/24 11.22.22.1 0 100 200 ?

*> 10.21.21.0/30 11.22.22.1 0 100 200 ?

*> 10.100.100.1/32 11.22.22.1 0 100 200 ?

rtr4#

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