Confirmation on BGP update-source config, basic lab

Answered Question
Jul 14th, 2009

Hi folks, can you clarify few questions I have? Please see attached diagram with detailed IP addressing and config.

Questions:

a) Don't I need to use

router bgp 213

neighbor 66.1.3.1 update-source loopback0

neighbor 66.1.3.1 ebgp-multihop

for all bgp statements since I am establishing

eBGP session? I included 'update-source and ebgp-multihop' commands

and everything works fine too, but I do not

understand how it works even without it?

b) You see that bgp sessions are being established using serial interface IPs. It

could be a better practice if loopback interfaces

were used for bgp sessions, right?

c) Can you explain again why I need to add

"ip route 65.7.128.0 255.255.192.0 Null0" to make my internal router get routes to Primary and

Backup routers?

I have this problem too.
0 votes
Correct Answer by k.hariharan1 about 7 years 4 months ago

ebgp multihop command is for non directly connected ebgp peer.

Gw router1(bgp)(AS 200)---->Switch(As200)--->router2 bgp(AS 300)

in the above setup while you configure ebgp session b/w router1 to router 2 , if u miss to configure ebgp multihop command , your bgp state will never come to established.

for the directly connected ebgp peer if u use update source as loopback and if u use ebgp multihop command , router will assume that loopback of remote router is in 2 hop.

hi guys, correct me if i am wrong

Regards,

Hariharan k

Correct Answer by Jon Marshall about 7 years 4 months ago

Marlon

a) No you don't. In fact generally speaking you use the physical interface addresses with EBGP and loopback addresses with IBGP.

So it will work fine with both physical and loopback addresses as you have discovered.

b) See a).

c) For an EBGP router to advertise a network using the "network" statement there must be a match in the IGP routing table for that route. So you have

network 65.7.128.0 mask 255.255.192.0

there must be a match in the routing table "sh ip route" for the router to advertise this to other EBGP peers.

As you had to add a static route to your config -

ip route 65.7.128.0 255.255.192.0 Null0

it's safe to assume that this route was not in the routing table originally and so it was not adervtised out until you added the static route.

Jon

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Correct Answer
Jon Marshall Tue, 07/14/2009 - 20:36

Marlon

a) No you don't. In fact generally speaking you use the physical interface addresses with EBGP and loopback addresses with IBGP.

So it will work fine with both physical and loopback addresses as you have discovered.

b) See a).

c) For an EBGP router to advertise a network using the "network" statement there must be a match in the IGP routing table for that route. So you have

network 65.7.128.0 mask 255.255.192.0

there must be a match in the routing table "sh ip route" for the router to advertise this to other EBGP peers.

As you had to add a static route to your config -

ip route 65.7.128.0 255.255.192.0 Null0

it's safe to assume that this route was not in the routing table originally and so it was not adervtised out until you added the static route.

Jon

Correct Answer
k.hariharan1 Tue, 07/14/2009 - 20:48

ebgp multihop command is for non directly connected ebgp peer.

Gw router1(bgp)(AS 200)---->Switch(As200)--->router2 bgp(AS 300)

in the above setup while you configure ebgp session b/w router1 to router 2 , if u miss to configure ebgp multihop command , your bgp state will never come to established.

for the directly connected ebgp peer if u use update source as loopback and if u use ebgp multihop command , router will assume that loopback of remote router is in 2 hop.

hi guys, correct me if i am wrong

Regards,

Hariharan k

Jon Marshall Tue, 07/14/2009 - 21:51

Hariharan

Just to clarify. If the switch is a L2 switch in your above example then you wouldn't need to use ebgp-multihop.

If the switch was a L3 switch then you would need to use the ebgp-multihop command.

Jon

chinkevi_2 Tue, 07/14/2009 - 21:35

for a and b, depends on your situation. If your bgp peer is in same subnet, it makes more sense to use serial address instead. If you are doing load balancing, it would be better to use loopback.

Loopback generally is preferred because it's always up, but in your case, bgp peering will die anyway if serial is down.

as for ebgp multihop, only if your peer is not directly connected.

as for c)

the static route is there so that bgp will advertise the route, regardless if your lan is down. Remember synchronisation rule and the condition bgp will install the network into bgp table?

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