Kevin Dorrell, CCIE #20765

15 Feb 2008

NMC Lab 7.7 – BGP

Filed under: BGP — dorreke @ 05:22

So on to the BGP, and this presented some real challenges.   So much so that it took up most of the afternoon.

7.7.1 – The first challenge was to peer AS100 on R1 and AS300 on R3 through VLAN 10.  This VLAN had two subnets; each router had a primary address and a secondary address with the rôles reversed at each end.  The peering was not allowed to refer to the secondary address.  This was actually a straightforward matter of each router connecting the other router’s primary, while sourcing from its own primary address.  In my usual manner I tried to make it a lot more complicated by using the loopback addresses, which of course were in different routing domains, so the connection went twice around the houses to get there.  “Keep It Simple Stupid”.

I’m proud to say I eventually did hit on the correct solution without looking it up in the AK.  What still I don’t understand is why the AK suggests you need ebgp-multihop for this peering.  Each router has only one hop to get to the partner’s primary address.  I have asked that on the DISCUSSiT forum, but no reply yet.  But several other people have commented on it in the past.  It rather makes 7.8.1. a null-requirement.

7.7.4. I didn’t understand how VLAN 21 and 22 should “destabilize” the BGP domain.  Even single link flapping gets added and taken away continuously, but that is hardly “destabilize”, especially with the delays inherent in BGP.  The solution I would have gone for was the dampening – I wouldn’t have thought of the aggregate.  In any case, the aggregate doesn’t protect you from both flapping together, which is what the question seemed to imply.

But I am having some problems configuring the dampening.  I keep getting an error message, and I cannot work out where if have gone wrong:

router bgp 300 
 no synchronization 
 bgp router-id 172.16.103.1 
 bgp log-neighbor-changes 
 bgp dampening route-map BGP-damped 
 network 172.16.13.0 mask 255.255.255.0 
 network 172.16.31.0 mask 255.255.255.0 
 neighbor 172.16.31.1 remote-as 100 
 neighbor 172.16.31.1 transport connection-mode active 
 neighbor 172.16.31.1 update-source FastEthernet0/0.10 
 neighbor 172.16.36.6 remote-as 600 
 neighbor 172.16.102.1 remote-as 300 
 neighbor 172.16.102.1 update-source Loopback103 
 neighbor 172.16.105.1 remote-as 300 
 neighbor 172.16.105.1 update-source Loopback103 
 no auto-summary 
: 
ip prefix-list BGP-damped seq 5 permit 172.16.21.0/24 
ip prefix-list BGP-damped seq 10 permit 172.16.22.0/24 
: 
route-map BGP-damped permit 10 
 match ip address prefix-list BGP-damped      

:      

Jan  5 23:49:54.144: %BGP-3-BADROUTEMAP: Bad parameters in the route-map BGP-damped applied for Dampening

7.7.6. For some reason I wasn’t aware of the command bgp default local-preference.  Some time I’m going to have to read the BGP command reference from cover to cover; there are so many geek-knobs.  there seems to be a slight difference between using this command and setting the local-preference in a route-map on the incoming EBGP routes: the local router does not see the local-preference.  That is, the command applies to the routes outgoing to the IBGP peers.

7.8.2. is about using a peer-session template.  I included the remote-as command in the template as well, and it seems to work OK.  I wonder why the SHOWiT does not do that.

By the way, where does it say in the scenario that CAT1 Lo110 does not have to be reachable from the other routers?

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