IP ROUTING

[fmugambi]

Hello guys, below is my topology,

I have added internet fw  and introduced ISP on site B.

Before site B resources/ servers would go via mpls to site a to access internet.

I introduced the above to make each site use its own isp respectively.

I ran to challenges, site b resources were not able to get to their isp/internet, until i introduced " ip route 0.0.0.0/0 sitebinternetfw interface. --> which works but causes some issues.

is there a way, to manipulate just internet traffic for site b resources without introducing static entries? and not affecting site a and b from communicating with each other?

[fmugambi]

no, reason is i wanted to completely bring out the challenge i have in prod network.

where, i have alot of static routes between "r5 and r6", but ospf redistributes them to bgp in r3.

do i have to add those very many static entries/ subnets as prefix-list on bgp. bgp wont advertise anything not on the prefix-list, as much as the routes are redistributed by ospf process on bgp?

[MHM Cisco World]

do i have to add those very many static entries/ subnets as prefix-list on bgp. bgp wont advertise anything not on the prefix-list, as much as the routes are redistributed by ospf process on bgp? Yes 

[MHM Cisco World]

do i have to add those very many static entries/ subnets as prefix-list on bgp. bgp wont advertise anything not on the prefix-list, as much as the routes are redistributed by ospf process on bgp? Yes 

[fmugambi]

added, this

and was able to learn the route on r1.

whats the risk of using it in a production env? if i manage both sites?

assume r1 is border g/w for site a and r3 is border g/w for site b.

[MHM Cisco World]

Add this is same of not using route-map at all 

This prefix list line allow all prefix to advertise between two bgp.

MHM

[fmugambi]

this brings me to the next question,

if i had static route 172.16.57.0/24 on r3, then redistribute static under bgp config, even without adding 172.16.57.0/24 on prefix-list,

r1 is able to learn about this route.

why does this not happen with dynamic routes redistribution on bgp?

[fmugambi]

when i have certain config,

does it mean the seq 1285 wont work as already seq 1000 is denied?

[MHM Cisco World]

You allow all prefix in prefix list that match in route - map line 10 so the router dont have any thing to match for route map line 1000

I.e. It useless 

MHM

[fmugambi]

i have same setup now on prod, but bgp speaking routers are 1941s.

have put 0.0.0.0/0 le 32. but site b does not learn some static learned routes on site a.

what would be the issue?

but when i add a static route on site 1941 to downstream router as next hop, site b is able to learn this route and vice-versa.

why is that so?

and do you mean the routes out deny has no impact ?

[MHM Cisco World]

and do you mean the routes out deny has no impact ? If you use 0.0.0.0/0 le 32 in any line above deny then deny have no impact.

[MHM Cisco World]

have put 0.0.0.0/0 le 32. but site b does not learn some static learned routes on site a.

what would be the issue? 

In site A

Did you see static route in show ip bgp ?

That first point ypu need to check.

Also in Prod did you use same as lab redistrubte static into ospf and the  redistribute ospf into bgp ?

[fmugambi]

see, the deny statement is not on prefix-list sequence, it just under route map command,

so as attached whats the order of operation?

[MHM Cisco World]

There is no relation between Seq of prefix list and Seq of route-map

The process as below

1- Router check line 10 of route-map 

The line have match prefix-list X 

Router now check all line of prefix-list X 

If there is match then route will advertise since this line is permit 

Note:-you use prefix list 0.0.0.0/0 le 32 so all prefix will match prefix list X of line 10

2- Router check then route-map line 1000 

This line have no match so by defualt it match ALL prefix and action is dont advertise' but all prefix already match by above route-map 10 prefix list X so it have no impact 

[fmugambi]

this is understood.

question , i replaced "redistribute ospf 30 match internal external 1 external 2" with "redistribute ospf 30" on r3 and noticed as much as r3 knows dest 172.16.57.0/24 via external ospf, it does not redistribute it to bgp, therefore r1 no longer knows how to get to 172.16.57.0/24 network.

before,

after,

is this expected, is this normal. cause i though " redistibute ospf 30" redistributes everything without filtering, policies and all as compared to "redistribute ospf 30 match internal external 1 external 2" ?

[MHM Cisco World]

Redistrubte ospf without match in ex1 ex2

Will make ospf only redis internal (any prefix inject into ospf via network command under ospf process) by defualt other external( any prefix inject i to ospf via redis) will not redis into bgp.

So you need match.

You can use match with route-map to make more filter which in ex1 ex2 prefix you want to redis into bgp from ospf

MHM

[fmugambi]

so if have alot of static subnets, i need to define them in prefix-list with permit, then put them under route-map?

if i have alot do i use 0.0.0.0/0 le 32?

what if i just use match ex1 and ex 2?. since am in control of the downstream static routing?