Never
There are two typical scenarios when people carry default route in dynamic routing protocol, I'll address these separately and explain why you shouldn't do it, and what you should do instead.
This is probably the most common scenario, maybe you're giving your customer default route, maybe it's your own firewall or really any situation where neighbor won't carry full routing table and neighbor isn't strictly same administrative domain.
Problem with default route here is, that if your PE gets disconnected from core, you're still originating the default route and CE is unaware of this and you're blackholing customer traffic until BGP is manually shutdown. You could conditionally advertise default, but that is just useless overhead, instead of default you should advertise to CE any aggregate route which is originated from multiple core boxes, such as your PA aggregate, or really any stable route originated from multiple places, but not local PE.
Customer would just add this to their router:
Lately, I've been playing around with DHCPv6 and SLAAC on my home network.
When configuring IPv6 addresses on the network interfaces there are three ways of doing this. We can use Stateless address autoconfiguration (SLAAC), DHCPv6 (statefull) or we can configure the address manually. SLAAC is by far the easiest way to configure IPv6 addresses, simply because you don't have to configure any IPv6 address. The way it works is that the router on your network will advertise the IPv6 prefix (/64) using multicast (remember that with IPv6 there is no such thing as broadcast). The host will receive/request this prefix advertisement and will auto generate the last 64 bits to make a fully working IPv6 address. When auto generating the address the host will use it's mac address (which is 48 bits) and insert "ff:fe" in the middle of it. This is also known as EUI-64. One drawback of EUI-64 is that you're trackable on the Internet because the mac address will normally not change when using the same host (e.g. laptop, smartphone, tablet, etc..). To overcome this issue SLAAC has been extended with something called Privacy Extensions. When this is enabled the host part (last 64 Continue reading
IPv6 designers recognized that IPv4 header has several faults, these were addressed to a different degree. Particularly annoying was IPv4 options which caused TCP/UDP/ICMP data to shift, as it made IPv4 header length variable. IPv6 header is fixed length, there is 'next-header' option, which will instruct how to parse data after IP header. Typically 'next-header' would be TCP, UDP or ICMP, and rest of packet would be exactly like in IPv4 (apart from mandatory checksum in UDP).
Where the complexity (some might say design fault) is that 'next-header' could be any large number of more exotic extension header, each of which have 'next-header' field themselves. Standard does not specify any limitation how many headers you could have, so you need to be able to parse packet up-to MTU length. The final extension header typically would contain TCP/UDP/ICMP and normal IPv4 style packet would follow.
Unfortunately no practical router has MTU wide view to the packet, you have 64B, 128B or 256B view, after which you are completely unaware of the packet content, it's just bits in memory which you cannot process in any meaningful way. Your PC won't have same problem, it does not have specialized hardware to quickly forward Continue reading
Imagine a group of researchers planning to speak at a conference regarding a previously undiscovered vulnerability present in most homes that would allow a thief to rob your home of its valuables with complete ease. You would probably be interested in hearing what they had to say so you could take the necessary precautions to protect your home.
Now imagine when they presented their findings, they went on to state that it was incredibly easy to do, so long as you left your front door open and also provided them with the security code for any alarm systems. You would probably find this implausible and simply the proliferation of fear, uncertainty, and doubt.
That's precisely what happened last week at the well-respected Black Hat security conference in Las Vegas when researchers from the Israel Institute of Technology and Advanced Defense Systems, Ltd. presented their findings of a serious vulnerability present in OSPF. So serious in fact, the researchers stated the only way to properly mitigate the threat, short of fixing the protocol, is to switch to another routing protocol such as RIP or IS-IS.
The researchers went on to present their research of two previously undiscovered methods Continue reading
The inevitable march towards merchant silicon for Ethernet switching is continuing with the announcement from Intel today that it is acquiring Fulcrum Microsystems. Fulcrum of course the silicon vendor that is the core of our low-latency switch family that is the most widely used switch across the world for high-frequency trading.
I wanted to share my thoughts on what this means for the industry going forward.
10 Gigabit Ethernet – the time is right:
First, virtually every new 10 Gigabit Switch announced this year is based on merchant silicon. We at Arista, of course, have been at the forefront of bringing multiple merchant switch architectures to market, but why all of the sudden this stampede?
The simple answer is that the technology advantages of merchant silicon in terms of throughput and cost-performance are so overwhelming compared to legacy platforms based on proprietary silicon designs, that merchant silicon is where the market is going.
New data centers that are built for the cloud require vastly more network scalability than the data center of yesterday. Throughput of servers has advanced at the speed of Moore’s law. The next generation of Intel server will have more than 100 times the throughput of the Continue reading
When using a routing protocol over GRE tunnels you might end up learning the tunnel endpoint via the routing protocol. When the tunnel endpoint is preferred using the route learned from the routing protocol you end up with a flapping tunnel.
The router will detect this and generates a message:
%TUN-5-RECURDOWN: Tunnel1 temporarily disabled due to recursive routing
The easiest way to solve this is by using a static route to the tunnel endpoint.
If this is not allowed you can also use a prefix filter. This filter should block the advertisement of the tunnel endpoint prefix via the GRE tunnel. For example:
ip prefix-list FILTER_TUNNEL_ENDPOINT seq 5 deny 150.1.10.0/24
ip prefix-list FILTER_TUNNEL_ENDPOINT seq 10 permit 0.0.0.0/0 le 32
This will match the tunnel endpoint address (150.1.10.0/24), but allow the other prefixes.
Use this prefix-list to filter the outgoing advertisements on the tunnel interface.
router rip
distribute-list prefix FILTER_TUNNEL_ENDPOINT out Tunnel1
ip local policy route-map <route-map>
route-map PBR_FROM_R3 permit 10
match ip address FROM_R3_TO_R4
set ip next-hop 155.1.0.5
set ip next-hop verify-availability
set ip default next-hop 155.1.146.4
route-map PBR_FROM_R3 permit 20
match ip address FROM_R3_TO_R5
set ip next-hop verify-availability 155.1.146.4 1 track 1
set ip default next-hop 155.1.0.5
Create one or more access-lists that specify what traffic should use policy routing.
ip access-list extended FROM_R4
permit ip host 155.1.146.4 any
ip access-list extended FROM_R6
permit ip host 155.1.146.6 any
Then create a route-map that will match the defined access-lists and specify an action.
route-map PBR permit 10
match ip address FROM_R4
set ip next-hop 155.1.13.3
route-map PBR permit 20
match ip address FROM_R6
set ip next-hop 155.1.0.5
route-map PBR permit 30
# will match any other traffic
Tie the route-map to an interface to enable policy routing.
interface FastEthernet0/0
ip policy route-map PBR
Usefull debug commands:
#debug ip policy