BGP Candidate Default Route Query

[jeetkulkarni]

We currently have head office and few branches with BGP running for site to site connectivity over the WAN. I am trying to get head around how the default route for bgp works. 

Below is a similar configuration of on one of our remote sites.

router bgp 6xx06
bgp router-id 1.1.1.1
bgp log-neighbor-changes
network 1.111.6.111 mask 255.255.255.255
redistribute connected
neighbor 1.1.111.1 remote-as 6xx01
neighbor 1.1.111.1 timers 8 24
neighbor 1.1.111.1 send-community
neighbor 1.1.111.1 soft-reconfiguration inbound
neighbor 1.1.111.1 weight 10
neighbor 1.1.111.1 prefix-list Site-routes-in in
neighbor 1.1.111.1 prefix-list Site-routes-out out
neighbor 1.1.111.1 route-map Site-link-in in
neighbor 1.1.111.1 route-map Site-link-out out

When we look at the - show ip route command below is the brief output from a remote site.

#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
+ - replicated route, % - next hop override

Gateway of last resort is 1.1.111.1 to network 0.0.0.0

B* 0.0.0.0/0 [20/0] via 1.1.111.1, 4w2d
10.0.0.0/8 is variably subnetted, 66 subnets, 4 masks
B 10.0.0.0/8 [20/0] via 1.1.111.1, 2w2d
B 10.150.0.0/16 [20/0] via 1.1.111.1, 4w2d
B 10.150.0.0/24 [20/0] via 1.1.111.1, 4w2d
B 10.150.1.0/24 [20/0] via 1.1.111.1, 4w2d
B 10.150.2.0/24 [20/0] via 1.1.111.1, 4w2d
B 10.150.3.0/24 [20/0] via 1.1.111.1, 4w2d

My doubt is how did the B* route come into play? How does 0.0.0.0 go via 1.1.111.1?

I checked the full running configuration but could not find out. Can you guys point me in the correct direction? Is this done by ISP?

[Meheretab Mengistu]

Hi,

---

@jeetkulkarni wrote:

We currently have head office and few branches with BGP running for site to site connectivity over the WAN. I am trying to get head around how the default route for bgp works. 

Below is a similar configuration of on one of our remote sites.

router bgp 6xx06
bgp router-id 1.1.1.1
bgp log-neighbor-changes
network 1.111.6.111 mask 255.255.255.255
redistribute connected
neighbor 1.1.111.1 remote-as 6xx01
neighbor 1.1.111.1 timers 8 24
neighbor 1.1.111.1 send-community
neighbor 1.1.111.1 soft-reconfiguration inbound
neighbor 1.1.111.1 weight 10
neighbor 1.1.111.1 prefix-list Site-routes-in in
neighbor 1.1.111.1 prefix-list Site-routes-out out
neighbor 1.1.111.1 route-map Site-link-in in
neighbor 1.1.111.1 route-map Site-link-out out

When we look at the - show ip route command below is the brief output from a remote site.

#show ip route

Gateway of last resort is 1.1.111.1 to network 0.0.0.0

B* 0.0.0.0/0 [20/0] via 1.1.111.1, 4w2d
10.0.0.0/8 is variably subnetted, 66 subnets, 4 masks

My doubt is how did the B* route come into play? How does 0.0.0.0 go via 1.1.111.1?

I checked the full running configuration but could not find out. Can you guys point me in the correct direction? Is this done by ISP?

---

It is advertised from the neighbor. Please share the output of 'sh run | s route-map|prefix-list. It could be found in the accepted routes configuration.

HTH,

Meheretab

[Georg Pauwen]

Hello,

in addition to Meheretab's post, post the full BGP configuration of the head office router. Look for any of the lines in bold (your IP addresses are different, obviously)...

router bgp 1
bgp log-neighbor-changes
network 1.1.1.1 mask 255.255.255.255
network 192.168.1.0 mask 255.255.255.252
network 0.0.0.0
redistribute static
neighbor 192.168.1.2 remote-as 2
neighbor 192.168.1.2 default-originate
default-information originate

ip route 0.0.0.0 0.0.0.0 192.168.1.1

[jeetkulkarni]

I cannot paste full configuration as of now but i found that the head office routes had this prefix entry. Probably that's the reason i think

ip prefix-list HO-routes-in description "Permit routes from HeadOffice"
ip prefix-list HO-routes-in seq 100 permit 192.168.0.0/16 ge 32
ip prefix-list HO-routes-in seq 110 permit 10.150.0.0/16
ip prefix-list HO-routes-in seq 120 permit 0.0.0.0/0 ??

[Georg Pauwen · Accepted solution]

Hello,

headquarters is sending the default route, one way or another. You cannot really tell how, but if you an produce the output of 'show ip bgp neighbors x.x.x.x received-routes', it gives you an indication:

R2#sh ip bgp neighbors 192.168.1.1 received-routes
BGP table version is 6, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

Network Next Hop Metric LocPrf Weight Path
*> 0.0.0.0 192.168.1.1 0 0 1 i
*> 1.1.1.1/32 192.168.1.1 0 0 1 i
* 192.168.1.0/30 192.168.1.1 0 0 1 i

The 'i' at the end means the default route is either sent with the 'network 0.0.0.0' command, or the 'neighbor x.x.x.x default-originate' command/

If you see a '?' instead of the 'i' in the Path field, that means the default route is sent by means of the 'default-information originate' command under the BGP process...