why bgp route has " <" missing

[sarahr202]

Hi everybody

Please consider the following scenario:

In the above scenario, BGP is being used between AS 2 and AS1

R1 and R2 are in AS2, while R4 is AS 1

R1 and R2 are running BGP and RIP

R4 has loopback 10.0.0.0/8 which is being advertised to R1 and R2 in AS 2

When I issue " show ip bgp" on R1, I get the following output:

R1#show ip bgp

BGP table version is 2, local router ID is 199.199.199.1

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

              r RIB-failure, S Stale

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

   Network          Next Hop            Metric LocPrf Weight Path

* i10.0.0.0         200.200.200.4            0    100      0 1 i

*>                  199.199.199.4            0             0 1 i

R1#

10.0.0.0  route has " .>"  missing.

When I issue " Show ip bgp 10.0.0.0" on R1 ,I get the following output:

R1#show ip bgp 10.0.0.0

BGP routing table entry for 10.0.0.0/8, version 2

Paths: (2 available, best #2, table Default-IP-Routing-Table)

  Advertised to update-groups:

        2

  1

    200.200.200.4 (metric 1) from 198.198.198.2 (200.200.200.2)

      Origin IGP, metric 0, localpref 100, valid, internal

  1

    199.199.199.4 from 199.199.199.4 (10.10.10.4)

      Origin IGP, metric 0, localpref 100, valid, external, best

The above output indicates there are two best routes to reach 10.0.0.0. If there are two best path available to reach 10.0.0.0 from R1,why Prefix 10.0.0.0 has " >" missing in the output of" show ip bgp" on R1.

===================================================

Configurations are given below:

R1#show running-config

Building configuration...

Current configuration : 1169 bytes

version 12.4

hostname R1

no aaa new-model

memory-size iomem 5

ip cef

no ip domain lookup

ip auth-proxy max-nodata-conns 3

-

interface Serial0/0

ip address 198.198.198.1 255.255.255.0

clock rate 2000000

!

interface FastEthernet0/1

no ip address

shutdown

duplex auto

speed auto

!

interface Serial0/1

ip address 199.199.199.1 255.255.255.0

clock rate 2000000

!

router rip

network 198.198.198.0

network 199.199.199.0

!

router bgp 2

no synchronization

bgp log-neighbor-changes

neighbor 198.198.198.2 remote-as 2

neighbor 199.199.199.4 remote-as 1

no auto-summary

R1#show ip route

Gateway of last resort is not set

R    200.200.200.0/24 [120/1] via 198.198.198.2, 00:00:27, Serial0/0

C    198.198.198.0/24 is directly connected, Serial0/0

C    199.199.199.0/24 is directly connected, Serial0/1

B    10.0.0.0/8 [20/0] via 199.199.199.4, 00:31:52

========================================

R2# show running-config

version 12.4

service timestamps debug datetime msec

service timestamps log datetime msec

no service password-encryption

hostname R2

boot-start-marker

boot-end-marker

no aaa new-model

memory-size iomem 5

ip cef

no ip domain lookup

ip auth-proxy max-nodata-conns 3

ip admission max-nodata-conns 3

multilink bundle-name authenticated

interface Serial0/0

ip address 198.198.198.2 255.255.255.0

clock rate 2000000

!

interface FastEthernet0/1

no ip address

shutdown

duplex auto

speed auto

!

interface Serial0/1

no ip address

shutdown

clock rate 2000000

!

interface Serial0/2

ip address 200.200.200.2 255.255.255.0

clock rate 2000000

!

interface Serial0/3

no ip address

shutdown

clock rate 2000000

!

router rip

network 198.198.198.0

network 200.200.200.0

!

router bgp 2

no synchronization

bgp log-neighbor-changes

neighbor 198.198.198.1 remote-as 2

neighbor 200.200.200.4 remote-as 1

no auto-summary

R2#show ip route

Gateway of last resort is not set

C    200.200.200.0/24 is directly connected, Serial0/2

C    198.198.198.0/24 is directly connected, Serial0/0

R    199.199.199.0/24 [120/1] via 198.198.198.1, 00:00:25, Serial0/0

B    10.0.0.0/8 [20/0] via 200.200.200.4, 00:31:44

============================================================

Thanks and have a nice weekend.

[Edison Ortiz · Accepted solution]

sarahr202 wrote:
When I issue " Show ip bgp 10.0.0.0" on R1 ,I get the following output:
R1#show ip bgp 10.0.0.0
BGP routing table entry for 10.0.0.0/8, version 2
Paths: (2 available, best #2, table Default-IP-Routing-Table)
  Advertised to update-groups:
        2
  1
    200.200.200.4 (metric 1) from 198.198.198.2 (200.200.200.2)
      Origin IGP, metric 0, localpref 100, valid, internal
  1
    199.199.199.4 from 199.199.199.4 (10.10.10.4)
      Origin IGP, metric 0, localpref 100, valid, external, best
The above output indicates there are two best routes to reach 10.0.0.0. If there are two best path available to reach 10.0.0.0 from R1,why Prefix 10.0.0.0 has " >" missing in the output of" show ip bgp" on R1.

No, the output indicates there is only one best route which I bolded for you.

If you review this document:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

You will see the tie breaker is the 7th rule from the algorithm

Regards,

Edison

[sarahr202]

Hi Edison

One thing still puzzles me.  

Let say   R1 is receiving prefix  199.199.199.0 from  ospf, and rip

Since AD of ospf is lower than that of rip, R1 chose prefix advertised by ospf.

In our case R1 is receiving 10.0.0.0/8 from R4 via EBGP which has AD of 20 ,R 1 is also receiving the same prefix from R2 via iBGP which has AD of 200.

Routers use the concept of AD when receiving prefix from multiple routing protocols; routers use  metric/cost to choose the best path to reach a prefix being advertised by different  routers running  same protocol.

Since EBGP  has AD of 20 and IBGP has AD of 200, R1 will always prefer the lower AD  and thus routes advertised via EBGP will be chosen .

Then  why R1 has to check the list to decide which path  to choose to reach 10.0.0.0, R1 should simply look at AD  of EBGP and and choose the prefix advertised by EBGP without even looking at the list .?

Your expert advised is greatly sought.

Thanks and happy holidays to you.

[Edison Ortiz · Accepted solution]

You are correct. Routes are compared based on Administrative Distance or Prefix Length.

The longest prefix length will be preferred

The lowest AD will be preferred

In your case, you have 2 contending routes learned via BGP. When both routes are learned via BGP, you rule out the Administrative Distance concept and follow the BGP Best Path Algorithm.

Regards,

Edison

[sarahr202]

HI Edison.

You are saying  since there is no contending routing protocols in our case thus  the concept of AD does not apply here.

But here is my confusion:

Will the prefix 10.0.0.0 advertised by EBGP neighbor R4 to R1,  regarded as EBGP advertised route?

Will the prefix 10.0.0 advertised by IBGP neighbor R2 to R1, regarded as IBGP advertised route?

If yes, then we have two contending routing protocols IBGP which has AD of 200 and EBGP which has AD of 20,both of them advertising prefix 10.0.0.0/8 to R1

I would realy appreciate your input.

Happy holidays!

[Edison Ortiz]

The routing protocol in question is BGP. There is no routing protocol called iBGP nor eBGP.

iBGP and eBGP are neighbor types and you never use Administrative Distance as a factor when comparing routes between eBGP and iBGP. As you stated, the AD will be used when comparing BGP with other IGPs if contending routes have identical prefix lengths.

Regards,

Edison

[sarahr202]

Hi Edison

My question is If IBgp anf EBgp are not protocols and they are just terms to define bgp neighbors then why do we have ADs assigned to them as indicated below:

Routing protocol /Route source	Default Distance Values
Connected interface	0
Static route	1
Enhanced Interior Gateway Routing Protocol (EIGRP) summary route	5
External Border Gateway Protocol (eBGP)	20
Internal EIGRP	90
IGRP	100
OSPF	110
Intermediate System-to-Intermediate System (IS-IS)	115
Routing Information Protocol (RIP)	120
Exterior Gateway Protocol (EGP)	140
On Demand Routing (ODR)	160
External EIGRP	170
Internal BGP	200
Unknown	255

Thanks and have a nice weekend.

[Edison Ortiz · Accepted solution]

You can make the same argument for External EIGRP and Internal EIGRP, are they different routing protocols?

[sarahr202]

In case of external eigrp ( Ad 170)  eigrp (ad 90) and both advertising same prefix, Router does use the concept of AD.

What I don't understand is why the router does not use the concept of AD when receiving  same prefix from ebgp( ad 20) and ibgp (ad 200).

Thanks and I really appreciate your help..

Happy holidays!.

[Edison Ortiz · Accepted solution]

The rule is to use the 'Best Path Algorithm' (URL posted before) when routes are learned via BGP.

By following this rule, you can influence routing decision on BGP speaking routers w/o depending solely on the Administrative Distance. For instance, if you want to choose an iBGP learned route over an eBGP learned route, you can use any of the 6 previous steps by using the BGP metrics such as WEIGHT, LOCAL_PREF, AS_PATH among others.

I've had to use these metrics extensively during designs for better traffic engineering.

Regards,

Edison

[sarahr202]

Thanks Edison

I apologize for this being stubborn.

Here is gist of what I have learned thanks to you.

The concept of AD applies when router has to choose a same prefix advertised by different contending protocols or some times  same protocol  such as EIGRP(  external eigrp  ad 170. eigrp 90).

However in  the case of BGP where BGP is the only routing protocol, the concept of AD does not apply unlike EIGRP.

Like EIGRP, BGP does have different AD, (Ibgp 200,ebgp 20). But these AD were not taken into account by router when choosing a same prefix advertised by Ebgp peer and IBGP peer. In such situation, router just follow the " The best  path algorithm".

thanks and happy holidays

[Edison Ortiz]

Correct.

Per your diagram above, you can influence R1 to go via R2 in order to get to R4 even if the route learned from R2 is iBGP (AD 200).

You can change the local_pref for 10.0.0.0/8 in R2 when the route is being learned from R4.

After making the change, examine the routing table as well as the bgp table in R1 and you will see the iBGP route chosen as preferred.

You can use other metrics such as WEIGHT or AS_PATH Prepend on R1 to come up with the same traffic engineering.

Again, any of those chances can be done without altering the Administrative Distance on iBGP nor eBGP.

Regards,

Edison